Method of Forming a Multilayer Substrate Comprising a Layer of Silicon and a Layer of Diamond Having an Optically Finished (or a Dense) Silicon-Diamond Interface

ABSTRACT

A method of making a multilayer substrate, which can include a silicon layer having an optically finished surface and a chemical vapor deposition (CVD) grown diamond layer on the optically finished surface of the silicon layer. At the interface of the silicon layer and the diamond layer, the optically finished surface of the silicon layer can have a surface roughness (Ra)≤100 nm. A surface of the grown diamond layer opposite the silicon layer can be polished to an optical finish and a light management coating can be applied to the polished surface of the grown diamond layer opposite the silicon layer. A method of forming the multilayer substrate is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.15/450,110, filed on Mar. 6, 2017, which claims the benefit of U.S.Provisional Patent Application No. 62/305,197, filed Mar. 8, 2016, thedisclosures of each of which are incorporated herein by reference intheir entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention concerns a method of making a substrate comprisinga layer of silicon and a layer of diamond having an optically finishedsilicon-diamond interface and, optionally, an optically finished diamondsurface. It also describes applications for said substrate.

Description of Related Art

Diamond is one of the hardest materials known, having a Mohs Hardness of10, which makes diamond most useful for applications such as cutting,machining, drilling, milling, etc. Diamond is also one of the mostthermally conductive materials known, having a thermal conductivity upto 2000 to 2200 watts per meter per Kelvin, which makes it a desirablematerial for applications in thermal management under demandingconditions. Diamond also has an extremely low coefficient of friction,which makes it a versatile material for uses such as brakes. Withdiamond on diamond, it is the low wear coefficient and lubricating usesunder extreme conditions which makes it advantageous. Diamond also hasdesirable optical material for transmitting microwave, infrared,visible, and other ultraviolet electromagnetic waves. Diamond also hashigh stability when used as detector for high fluence nuclear radiation.In addition, diamond is also a highly inert material in a chemicalenvironment that can involve strong acid, strong base, strong oxidizingagent, or strong reducing agent, even at elevated temperatures or atcryogenic conditions. Furthermore, diamond has a high refractive index,which leads to its use in the jewelry industry.

In addition, the acoustic velocity for diamond is 18,500 m/s, which ismuch higher than that for single crystal silicon (8,500 m/s) and forsilicon carbide (13,300 m/s), as shown in Table 1, below. Acousticvelocity is an intrinsic property of a material that is considered whenmaking an acoustic wave resonator. An acoustic resonator is widely usedas a RF filter, an RF ocillator, a RF MEMS switch, and a microsystem(sensor). The resonance frequency of an acoustic resonator is directlyproportional to the acoustic velocity of the material and inverselyproportional to the geometric dimension of the resonator. The resonator(with a given geometry) that is made of diamond resonates at a frequencythat is 2.2 times of that of a resonator made of silicon. Hence, for agiven resonant frequency, a resonator made of diamond can have ageometric dimension that is about 2.2 times smaller than the geometricdimension of a resonator made of silicon. A diamond resonator can alsohave a lower energy loss during vibration, as compared to a siliconresonator, which is measured by a quality factor, Q. The theoretical Qfor diamond is around 380,000, as compared to that for silicon, which isaround 36,000. The higher the Q value is, the lower the energy loss isduring vibrations.

TABLE 1 Comparison of Acoustic Velocities and Theoretically PredictedQuality Factors at 1 GHz and 3 GHz*. Material ν_(a) (m/S) Q Theory @ 1GHZ Q Theory @ 3 GHZ Diamond 18,500 380,000 380,000 SiC 13,300 320,000116,000 Quartz 5,720 39,000 13,000 Si 8,500 36,000 36,000 AlN 10,9708,700 3,400 *Ivan Rivera, RF MEMS Resonators for Mass SensingApplications (PhD Dissertation), February 2015.

Therefore, it is desirable for a diamond acoustic wave resonator to haveboth high quality diamond and a high quality diamond surface, whichfacilitates achieving a higher resonant frequency, a higher Q value (ora low power loss for low power consumption), or a higher frequency Qproduct.

Industrially, diamond can be grown in reactors in a process calledchemical vapor deposition (CVD), where suitable growth conditions can beachieved by microwave-enhanced plasma, tungsten hot-filament, DC-Jetplasma, laser-induced plasma, acetylene-torch, etc.

In many diamond applications, the surface of a substrate comprisingdiamond needs to be finished optically smooth for the purpose of use asa sound wave medium, or as a substrate to reflect light/electromagneticwaves, or conduct heat energy away from electronics, photonics, oroptoelectronics via certain type of bonding mechanisms such as brazingor gluing. Since diamond is one of the hardest materials known,polishing diamond can be a slow and expensive process in which asubstrate comprised of diamond undergoes stress and, at the same time,heat is generated. Moreover, diamond is also fragile and easy toshatter. The heat and stress that are generated during polishing of adiamond film can lead to delamination or partial delamination of adiamond film comprising a substrate.

There is a substantial mismatch in coefficient of thermal expansionbetween diamond (about 1×10⁻⁶ m/m-K) and silicon (about 3×10⁻⁶ m/m-K).Diamond film typically grows on a silicon substrate at an elevatedtemperature. At the end of the diamond film growth process, thesubstrate comprising a layer of diamond and a layer of silicon coolsfrom a diamond growth temperature to room temperatures. Diamond shrinksmuch less than silicon, which can lead to substantial stress between thediamond layer and the silicon layer, where diamond suffers a compressionstress and silicon suffers a tensile stress. Such CTE mismatch stressincreases with the geometric dimension of the substrate comprising alayer of diamond and a layer of silicon. When the stress reaches acertain level, the diamond layer can delaminate from the silicon layer.Often times, the delaminated diamond layer shatters into many irregularsmall pieces due to the compression stress, or the silicon layer, e.g.,silicon wafer, suffers cracking due to the tensile stress.

SUMMARY OF THE INVENTION

The substrate disclosed herein comprises a layer of silicon and a layerof diamond having an optically finished silicon-diamond interface (or adense diamond-silicon interface with a diamond grain density of 10⁹/cm²or greater) and, optionally, an optically finished diamond surface. Thesurface roughness (Ra) of the diamond surface at the silicon-diamondinterface can be ≤100 nm, ≤50 nm, ≤30 nm, ≤20 nm, ≤10 nm, or ≤5 nm. Thesurface roughness of the silicon surface at the silicon-diamondinterface can be ≤100 nm, ≤50 nm, ≤30 nm, ≤20 nm, ≤10 nm, ≤5 nm, ≤2 nm,or ≤1 nm. The surface roughness of the diamond surface opposite to theinterface can be ≤100 nm, ≤50 nm, ≤30 nm, ≤20 nm, ≤10 nm, or ≤5 nm. Thethickness of the diamond layer of said substrate can be ≤5 microns, ≤10microns, ≤20 microns, ≤50 microns, ≤100 microns, ≤200 microns, ≤500microns, or ≤2,000 microns. The thickness of the silicon layer of saidsubstrate can be ≥10 microns, ≥50 microns, ≥100 microns, ≥200 microns,≥500 microns, ≥2,000 microns, or ≥5,000 microns. The diameter of saidsubstrate can be ≥1″ (25.4 mm), ≥2″ (50.8 mm), ≥3″ (76.2 mm)≥, 4″ (101.6mm), ≥5″ (127 mm), or ≥6″ (152.4 mm). The overall thickness of saidsubstrate can be ≥50 microns, ≥100 microns, ≥150 microns, ≥250 microns,≥500 microns, ≥1 mm, ≥3 mm, or ≥5 mm.

The diamond layer can be doped or undoped, polycrystalline,nanocrystalline, or ultrananocrystalline. The silicon layer can be dopedor undoped, polycrystalline, monocrystalline, etc. The diamond qualitycan have Raman half-height-peak-width of ≤20 cm⁻¹, ≤15 cm⁻¹, ≤10 cm⁻¹,or ≤7 cm⁻¹.

Also disclosed are growth conditions for making the substrate disclosedherein comprising a layer of silicon and a layer of diamond having anoptically finished silicon-diamond interface (or a dense diamond-siliconinterface) and, optionally, an optically finished diamond surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an example substrate comprised of a layer ofsilicon and a layer of diamond having an optically finishedsilicon-diamond interface and, optionally, an optically finished diamondsurface;

FIG. 2 is flow diagram of a process of producing the substrate of FIG.1;

FIG. 3 is a schematic diagram of an example microwave plasma chemicalvapor deposition (CVD) system that can be used for depositing thediamond layer on the silicon layer of the substrate shown in FIG. 1;

FIG. 4A is a photograph of a diamond film that was grown conforminglyonto a silicon substrate that included the pattern “II-VI” formed in thegrowth surface of the silicon substrate;

FIGS. 4B and 4C are close-up views of the nucleation side and growthside of the diamond film shown in FIG. 4A;

FIG. 5 is a graph of Raman intensity versus wavelength showing Ramangraphs of the growth side center, growth side edge, nucleation sidecenter, and nucleation side edge of the diamond film shown in FIG. 4A;

FIG. 6 is an SEM photo of a silicon surface of a polysilicon substrateon which a diamond film is grown after a first seeding step thatincludes nucleation of diamond on the silicon surface in accordance withExample 6 herein;

FIG. 7 is an interferometer roughness measurement of the polisheddiamond growth surface of the diamond on silicon substrate of Example 6herein;

FIG. 8 is an interferometer roughness measurement of the diamondnucleation side surface of the diamond on silicon substrate of Example 6herein;

FIG. 9 is an SEM photo of the diamond nucleation side surface of thediamond on silicon substrate of Example 6 herein;

FIG. 10 is an interferometer surface contour measurement of the diamondgrowth surface of the diamond on silicon substrate of Example 6 herein;and

FIG. 11 are Raman spectra of the as-grown diamond surface and thepolished diamond surface on the silicon substrate of the diamond onsilicon substrate of Example 6 herein.

DESCRIPTION OF THE INVENTION

Various non-limiting examples will now be described with reference tothe accompanying figures where like reference numbers correspond to likeor functionally equivalent elements.

With reference to FIG. 1, in an example, a composite substrate 2comprises a layer of silicon 4 and a layer of diamond 6, wherein aninterface 8 between the silicon layer and diamond layer can be opticallyfinished or physically dense with a substantially absence of voids orgaps among the diamond grains. More specifically, a diamond surface 10at the silicon-diamond interface 8 can be optically finished (can becharacterized by removing the silicon layer 4 allowing the diamondinterface surface to be exposed), or physically dense with a substantialabsence of voids or gaps among the diamond grains. The surface roughness(for the optically finished interface) (Ra) of the diamond surface 10 atthe silicon-diamond interface can be ≤100 nm, ≤50 nm, ≤30 nm, ≤20 nm,≤10 nm, or ≤5 nm. For a lapped, grounded or etched silicon surface atthe interface, the diamond-silicon interface can be characterized by thepresence of voids or gaps among the diamond grains, which can becharacterized by the diamond grain density. The diamond grain densitycan be ≥10⁴/cm², ≥10⁵/cm², ≥10⁶/cm², ≥10⁷/cm², ≥10⁸/cm², or ≥10⁹/cm².The surface roughness (Ra) of the silicon surface (for the opticallyfinished silicon) at the silicon-diamond interface can be ≤100 nm, ≤50nm, ≤30 nm, ≤20 nm, ≤10 nm, ≤5 nm, ≤2 nm, or ≤1 nm. The surfaceroughness, Ra, for a silicon surface at the silicon-diamond interface 8can be determined by the roughness of the same silicon surface prior todiamond growth.

For applications of substrate 2 in an acoustic wave resonator, a denselayer of diamond at the silicon-diamond interface allows the acousticwave to transport with a minimal scattering. Otherwise an acoustic wavewould be damped in a porous media.

The optically finished silicon-diamond interface also allows precisioncontrol of the geometric dimension of the resonator, which in-turncontrols an intrinsic acoustic resonance frequency (a critical parameterfor a RF communication filter, or oscillator, etc.).

For applications of substrate 2 in light wave management, a dense layerof diamond at the silicon-interface avoids light scattering and anoptically finished surface allows precision control of light reflection,refraction, and transmission. For applications of substrate 2 in thermalmanagement, a dense diamond at the silicon-diamond interface avoids slowheat conduction in the gas phase or in vacuum in the voids or gaps amongthe diamond grains, taking an advantage of fast phonon transport indiamond media, achieving high performances of thermal management.

Optionally, diamond surface 14 (opposite the silicon-diamond interface8) of substrate 2 can be optically finished. The optically finisheddiamond surface 14 can be achieved by a polishing process. The surfaceroughness (Ra) of diamond surface 14 can be ≤100 nm, ≤50 nm, ≤30 nm, ≤20nm, ≤10 nm, or ≤5 nm. The optical finishing of the diamond surface 14can be a factor in optical management for light control, thermalmanagement for bonding to a heat source, acoustic wave management forprecision control of geometric dimensions, and for buildingpiezoelectric and electrode structures for achieving desirable acousticwave resonance control and performance.

The silicon surface 16 (opposite the silicon-diamond interface 8) ofsubstrate 2 having an optically finished (or dense) interface, and,optionally, an optically finished diamond surface 14, can be ground,etched, or optically polished.

In an example, the diamond layer 6 of substrate 2 can be of anythickness. In an example, the thickness of diamond layer 6 can be ≤5microns, ≤10 microns, ≤20 microns, ≤50 microns, ≤100 microns, ≤200microns, ≤500 microns, or ≤2,000 microns. In an example, the thicknessof diamond layer 6 can be dictated by the precision needed in acousticwave resonance, light transport control and/or efficiency in thermalmanagement.

In an example, the silicon layer 4 of substrate 8 can be of anythickness. In an example, the thickness can be ≥10 microns, ≥50 microns,≥100 microns, ≥200 microns, ≥500 microns, ≥2,000 microns, or ≥5,000microns. The thickness of the silicon layer can be dictated by the needsof applications and the practical needs during the process offabricating substrate 2.

Substrate 2 can have any geometric (largest) dimension. In an example,the largest dimension (e.g., diameter) can be ≥1″ (25.4 mm), ≥2″ (50.8mm), ≥3″ (76.2 mm), ≥4″ (101.6 mm), ≥5″ (127 mm), or ≥6″ (152.5 mm). Thediameter of substrate 2 can be selected and controlled so it can behandled in different fabrication processes.

Substrate 2 can have a thickness of ≥50 microns, ≥100 microns, ≥150microns, ≥250 microns, ≥500 microns, ≥1 mm, ≥3 mm, or ≥5 mm. Thethickness of the substrate can be dictated by the fabrication process tomeet specific application needs.

The diamond layer 6 of substrate 2 can be undoped or doped, p-type orn-type, of a quality with Raman half-height-peak-width of ≤20 cm⁻¹, ≤15cm⁻¹, ≤10 cm⁻¹, or ≤7 cm⁻¹. The diamond layer 6 can be polycrystalline,nanocrystalline, or ultrananocrystalline Doped and conductive diamondlayer 6 can be used for a capacitive-drive acoustic resonance device.Doped and conductive diamond layer 6 can be used as an electrodematerial for high-frequency high-Q resonance media for acoustic wavemanagement. The quality of diamond layer 6 can also be selected toachieve a desired level of intrinsic acoustic velocity and low powerloss in acoustic resonance. The quality of diamond layer 6 can alsoeffect light wave absorption and the thermal conductivity, dictated bythe needs in optical and thermal applications.

The silicon layer 4 of substrate 2 can be undoped or doped, p-type orn-type, monocrystalline or polycrystalline. For single crystal silicon,silicon layer 4 can be cut from any crystal directions. The type ofsilicon comprising silicon layer 4 can also be selected for acousticwave management. In an example, a high-electric-resistance silicon layer4 may be needed. In another example, an electrical conductive siliconlayer 4 may be needed. Finally, in another example, a single or polycrystalline silicon layer 4 may be needed.

The growth of diamond layer 6 of substrate 2 can be performed by anydiamond growth process, including, but not limited to, microwaveplasma-enhanced chemical vapor deposition (CVD), DC-Arc-Jet-plasma CVDprocess, hot-filament CVD process, laser-induced plasma CVD process,acetylene-torch CVD process, etc.

Substrate 2 can have an aspect ratio of ≥5, ≥10, ≥15, ≥20, or ≥30, wherethe aspect ratio is defined as the ratio of the longest or largestgeometric dimension of substrate 2 (such as diameter) over the thicknessof substrate 2. In an example, the larger the aspect ratio of substrate2, the more difficult it is to make said substrate 2, particularlygiving the constraints in the fabrication process and the level of CTEmismatch between diamond 6 and silicon 4 materials.

The process for producing substrate 2 comprising a layer of silicon 4and a layer of diamond 6 having an optically finished (or dense)silicon-diamond interface 8 and, optionally, an optically finisheddiamond surface 14 is shown in the flow diagram of FIG. 2.

With reference to FIG. 2, in step S1, the surface 12 (for diamond filmgrowth) of the silicon substrate 4 can be ground or lapped, chemicallyetched, or optically polished with a chemical-mechanical polishingslurry. In step S2, the silicon surface 12 can be seeded withconventional diamond powders (having a particle size ranging fromsub-microns to tens microns) by ultrasonic treatment of the siliconlayer 4 with a slurry made out of the diamond particles and a liquidmedia (such as water, methanol, ethanol, isopropanol, or any liquidmedia) or gentle rubbing without generating deep surface scratches. Theseeding can also be achieved by a BIAS-enhanced seeding method.

In step S3, the silicon substrate 4 can then be placed inside a CVDgrowth system (such as the CVD reactor shown schematically in FIG. 3).At step S4, the diamond growth process can be started and, at step S8,can continue for a period of time to attain a desired thickness anddensity of small diamond crystals that bond to the surface of thesilicon substrate.

Once a desired thickness and density of diamond growth is complete, instep S9 the growth of diamond layer 6 is stopped and substrate 2,comprising silicon layer 4 and diamond layer 8 is unloaded from the CVDgrowth system. In steps S10 and S11, the growth surface of diamond layer6 can be optionally lapped (S10) and optionally polished (S11) to anoptical finish. As steps S10 and S11 are optional, these steps can beomitted if desired. If steps S10 and S11 are omitted, the process forproducing substrate 2 can advance from step S9 directly to step S12 asshown by dashed line 18 in FIG. 2. As shown in step S12, substrate 2comprising a layer of silicon 4 and a layer of diamond 6 has anoptically finished silicon-diamond interface. Substrate 2 can also havea dense diamond-silicon interface with substantial absence of voids orgaps among the diamond grains. Finally, substrate 2 can optionally havean optically finished diamond (nucleation) surface 14.

As shown in FIG. 2, the process for producing substrate 2 can alsoinclude, between steps S4-S8, steps S5-S7. In step S5, the CVD diamondgrowth process can be optionally stopped and substrate 2 including aportion of diamond layer 6 CVD grown on silicon layer 4 in step S4 canbe removed from the CVD growth system. In step S6, the portion of thediamond layer 6 CVD deposited on silicon layer 4 in step S5 can bereseeded or seeded again with nanodiamond particles.

In step S7, substrate 2 including the portion of the diamond layer 6reseeded with nanodiamond particles in step S6 can be placed back intothe CVD growth system wherein additional portions of diamond layer 6 canbe grown atop of the reseeded portion of diamond layer 6 previouslygrown.

Because steps S5-S7 are optional, these steps can be omitted if desiredwhereupon, assuming steps S5-S7 are omitted, the method proceedsdirectly from step S4 to S8 as shown by dashed line 20 in FIG. 2.

Chemical vapor deposition of diamond by microwave plasma is well knownin the art. FIG. 3 shows a schematic diagram of an example microwaveplasma CVD growth system 102 that can be used to grow diamond layer 6 onsilicon layer 4. In use of CVD growth system 102, silicon layer 4 isfirst placed on a substrate holder 122 at the base of a CVD reactor 116of CVD growth system 102. Thereafter, a mixture of reactive gases 106,comprising hydrogen and methane, is flowed into the CVD reactor 116. Theflow rate of the mixture of reactive gases 106 is controlled by a massflow controller 108. Exhausted gas 110 flows out CVD reactor 116,typically to a vacuum pump 112. Microwave energy is typically generatedby a microwave source, e.g., a magnetron, 114 and is guided to CVDreactor 116 through a quartz window 118. Inside CVD reactor 116, themicrowave energy is converted into a plasma 120 which radicalizeshydrogen molecules of reactive gases 106 into hydrogen free radicals, aswell as methane molecules of reactive gases 106 into methyl freeradicals, methylene free radicals, methyne free radicals, and secondaryor tertiary free radicals that contain two or more carbons. At thebottom of CVD reactor 116 sits substrate holder or support 122 thatinitially supports silicon layer 4 on which diamond layer 6 grows.

While plasma 120 is on, the radicalized free radicals that containcarbon species bombard the surface of silicon layer 4, which leads tofixation of the carbon species by a mechanism called “hit and stick”.After a suitable length of time in the presence of plasma 120, diamondlayer 6 grows to a suitable thickness.

It is well known in the art that the concentration of hydrogen andmethane in the mixture of reactive gases 6 are parameters that affectdiamond growth, in addition to the growth temperature (which can bemeasured by an optical parameter 126). Desirably, the plasma's 120 sizeis tuned to a size that is large enough to cover the upward facingsurface of silicon layer 4. The microwave power and pressure inside ofCVD reactor 116 are substrate-size-dependent. A person skilled in theart should be able to tune plasma 120 to a proper size that is largeenough to cover silicon layer 4 of different sizes for the purposes ofgrowing quality diamond film 6.

This first step seeding (S2 in FIG. 2) helps achieve good adhesion ofdiamond film 6 on silicon substrate 4. The first step seeding (S2 inFIG. 2) leads to diamond at the silicon-diamond interface having somevoids and gaps among the diamond grains. In general, the voids or gapsamong the diamond grains at the silicon-diamond interface are notdesirable for a high frequency and high Q (low power loss) acoustic waveresonator, or for a controllable optical management (low scatter), foran effective thermal management (high heat transfer). To help avoid theformation of voids or gaps at the silicon-diamond interface, the processshown in FIG. 2 can optionally include the second step diamond seeding(S5-S7 in FIG. 2). In the second step of diamond seeding, the siliconsubstrate of the first step is taken out of the reactor after thediamond growth process is stopped. The silicon substrate can then beseeded again using one or more nanodiamond slurries and ultrasonictreatments. Such nanodiamond slurries can be made of nanodiamond powdersalong with a liquid medium, such as water, methanol, ethanol,isopropanol, or any inorganic/organic liquid media. Then, thenanodiamond-seeded silicon substrate can then be placed back into theCVD growth system (reactor), followed by re-starting the diamond growthprocess (S8 in FIG. 2) on the nanodiamond-seeded silicon substrate.Nanodiamond seeding can lead to formation of an optically finishedsilicon-diamond interface (if using an optically-finished siliconsubstrate), or a dense silicon-diamond interface and reduced gaps orvoids over substrate 2 comprising a layer of silicon 4 and a layer ofdiamond 6 grown without the second step diamond seeding (S5-S7). Diamondfilm growth (S8 in FIG. 2) then continues inside the CVD growth systemuntil a desirable thickness of diamond film 6 is achieved.

In an example, it was observed that one step diamond seeding withnanodiamond powders alone leads to weak adhesion of diamond film 6 to anoptically-finished silicon substrate 4, leading to diamond film 6delamination or partial delamination after the CVD growth system is shutdown or during polishing in which substantial heat and stress areapplied on substrate 2 comprising diamond layer 6 and silicon layer 4.Such adhesion and delamination problems get magnified as the diameter ofsubstrate 2 increases.

The two-step diamond seeding described above allows for stronger bondingat the silicon-diamond interface 8 that can withstand the heat andstress resulting from post-diamond-surface-polishing, avoiding thediamond layer 6 from delaminating from the silicon layer 4. At the sametime, in the case of using a silicon layer 4 having an opticallyfinished surface 12, an optically-finished silicon-diamond interface canbe achieved for substrate 2 and, optionally, an optically finisheddiamond growth surface 14.

In the case of using a ground/lapped or chemically etched siliconsurface 12, the diamond nucleation surface 10 at the silicon-diamondinterface 8 can be dense with minimal voids or gaps among diamondgrains, evidenced by a diamond grain density at the silicon-diamondinterface 8 of ≥10⁴/cm², ≥10⁵/cm², ≥10⁶/cm², ≥10⁷/cm², ≥10⁸/cm², or≥10⁹/cm². After substrate 2 is taken out of CVD reactor 116, diamondgrowth surface 14 can be optionally lapped (S10 in FIG. 2), and,optionally, optically polished (S11 in FIG. 2) whereupon the thicknessof silicon layer 4 can be reduced to a desirable thickness.

The examples described herein use a silicon layer 4 that can be large inone dimension (diameter). The coefficient of thermal expansion forsilicon is about 3.0×10⁻⁶ meter/meter-Kelvin, while the coefficient ofthermal expansion for diamond is about 1.0×10⁻⁶ meter/meter-Kelvin.Moreover, diamond typically grows at elevated temperatures. Duringshutting down of CVD diamond growth, the temperature drops substantiallyfrom diamond growth temperature to room temperatures. The CTE mis-matchbetween diamond layer 6 and silicon layer 4 leads to substantial stressbetween diamond layer 6 and silicon layer 4. This can be severe whensilicon layer 4 is large in dimension. In addition, unlike metallicsubstrates such as tungsten and molybdenum, silicon is brittle andcannot survive plasma start-up during CVD growth of diamond layer 6 onsilicon layer 4. In an example, silicon wafers (6″ (152.4 mm) indiameter and 625 microns in thickness) were tried as silicon layers 4for diamond growth. Unfortunately, these silicon wafers shattered everytime during the early stage of the plasma 120 tuning process in CVDreactor 116. It was observed that only a piece of thick silicon layer 6survived the plasma start-up tuning process. In an example, a siliconlayer 4 that can survive the plasma start-up tuning process is ≥1 mm inthickness, ≥2 mm in thickness, ≥4 mm in thickness, ≥6 mm in thickness,or ≥8 mm in thickness.

As shown in FIG. 1, silicon layer (wafer) 4 can be bonded using abonding reagent 15 to a thicker substrate 17, such as, for example,another silicon substrate, a graphite substrate, a metallic substrate, aceramic substrate, or a glass substrate, etc. By bonding on to thickersubstrate 17, silicon layer 4 can survive the plasma start-up withoutshattering, or bowing up in the center, or “potato-chip” deformation.

The bonding reagent 15 can comprise a polymer, a carbon-based material,a silicate or silicate-based material, a metallic based material such assilver, gold, platinum, nickel, or copper, etc.

The silicon layer 4 and/or the diamond layer 6 of substrate 2 can bepartially removed, e.g., etched away. In case of partial removal ofsilicon layer 4, the diamond surface 10 at the silicon-diamond interface8 can be exposed. Such diamond surface 10 can be dense with minimal orno voids or gaps among the diamond grains. If the silicon surface 12 isoptically finished, the diamond surface 10 in the silicon-diamondinterface 8 can also have an optically finish having a surfaceroughness, Ra, ≤100 nm, ≤50 nm, ≤30 nm, ≤20 nm, ≤10 nm, or ≤5 nm.

In another example, diamond layer 6 can be used as a substrate orwindow. The diamond layer 6 can have a 1.06 micron light scatteringcoefficient (at a distance of 34 cm from a blocking lens) of ≤20/cm,≤15/cm, ≤10/cm, ≤7/cm, or ≤5/cm, which can be desirable for opticalapplications, thermal management applications, acoustic managementapplications, etc.

In another example, diamond layer 6 can have a diamond nucleationdensity ≥1.0×10⁵/cm², or ≥1.0×10⁶/cm², or ≥1.0×10⁷/cm², or ≥1.0×10⁸/cm²,or ≥1.0×10⁹/cm², which can also be desirable for transporting acousticwaves for acoustic management, transporting phonons for thermalmanagement, transporting photons for light management. Said diamondnucleation density can also be desirable for mechanical applications.Said diamond nucleation can result in diamond film 6 having a lowporosity for chemical inertness, and properties favorable for surfacefriction controls. Another example can be to polish the as-grown diamondsurface 14 while diamond layer 6 is still on silicon layer 4. In thiscase, the total thickness of diamond layer 6 and silicon layer 4 isthick enough to allow a conventional polishing process to hold andpolish substrate 2 without shattering diamond layer 6 during thepolishing process. After achieving an optical finish on surface 14 ofdiamond layer 6, substrate 2 can go through a thinning process (on thesilicon layer 4 side). At the end of this process, a thin piece ofsubstrate 2 comprising silicon layer 4 and diamond layer 6 having anoptically finished (or dense) silicon-diamond interface 8 and,optionally, an optically finished diamond surface 14 can be produced.

In another example, after producing substrate 2 comprising silicon layer4 and diamond layer 6 comprising an optically finished (or dense)silicon-diamond interface 8 and, optionally, a optically finisheddiamond surface 14, one or more light management coatings 19 such as,for example, an antireflective coating, a beam splitter coating, a totalreflective coating, etc., or a layer of piezoelectric material alongwith certain type of electrode layer(s) for acoustic wave management,can be applied to any exposed surface, e.g., on surface 14 of diamondlayer 6 as shown in FIG. 1. Such substrate 2 can also be further cutinto different geometric dimensions for various applications.

The diamond layer 6 of substrate 2 can be of optical quality, having alow absorption of electromagnetic waves such as infrared light, nearinfrared light, visible light, or UV light, having an absorbance of≤0.5/cm. It can also have a low loss tangent for microwave applications,for example a loss tangent of ≤1×10⁻². It can also be mechanical and/orthermal grade diamond (which could be dark, having a thermalconductivity of, for example, >800 watt/m-K). It can also be detectorgrade diamond (having a charge collection distance of ≥100 micron) orelectrochemical grade diamond (which requires doping with, for example,boron).

The diamond layer 6 of substrate 2 can be CVD grown via microwave-aidedplasma CVD process, a hot-filament CVD process, a thermal spray CVDprocess, an arc discharge plasma CVD process, a direct current thermalplasma CVD process, a radio-frequency plasma CVD process, a water-basedplasma CVD process, an acetylene touch plasma CVD process, a very highfrequency plasma CVD process, etc.

The growth temperature of diamond layer 6 of substrate 2 can range from600° C. to 1300° C. The growth rate for diamond layer 6 can be submicronper hour to 20 microns per hour. The methane concentration of thereactive gases 106 for growing diamond layer 6 can range from less than1% to as high as 5% in hydrogen. Other additives of the reactive gases106 can include oxygen, carbon monoxide, carbon dioxide, nitrogen,boron, etc. These other reactive gases additives can be added for thepurpose of diamond growth rate control and/or diamond quality control.

The silicon layer 4 for diamond layer 6 growth, can be ≥30 mm indiameter, ≥2″ (50.8 mm) in diameter, ≥66 mm in diameter, ≥3″ (76.2 mm)in diameter, ≥4″ (101.6 mm) in diameter, ≥5″ (127 mm) in diameter 1 or≥6″ (152.4 mm) in diameter.

The thickness of the silicon layer 4 of said substrate 2 can be, ≥1 mm,≥2 mm, ≥4 mm, ≥6 mm, or ≥8 mm.

The surface 16 of silicon layer 4 of substrate 2 can be opticallyfinished, chemically etched, and/or mechanically finished, such aslapping and/or grinding. The surface roughness, Ra, of the opticallyfinished surface 16 can be ≤20 nm, ≤15 nm, ≤10 nm, ≤5 nm, or ≤2 nm.

In another example, silicon surface 12 can optionally include a thinlayer of dielectric film 13, such as, for example, SiO₂, SiN, or one ormore other dielectric layers. Silicon substrate can be a silicon oninsulator (SOI) wafer. The dielectric film 13 can be any thickness. Inan example, dielectric film 13 can have a thickness suitable for aparticular function such as, for example, supporting diamond film 6 toachieve a particular function as acoustic resonator for acoustic wavemanagement. In an example, the thickness of dielectric film 13 can bebetween a few angstroms and 100 microns, between 1 nm and 50 microns,between 10 nm and 20 microns, or between 50 nm and 10 microns.

Each of the various example composite substrates 2 of diamond layer 6and silicon layer 4 can be used in one or more the followingapplications: optical applications for light/electromagnetic wavemanagement; as a substrate 2 for thermal management for electronics,photonics, and optoelectronics; as a substrate 2 for uses involvingchemical inertness; as a substrate 2 for making an acoustic waveresonator including a surface and or bulk acoustic resonator; as a RFfilter, an RF oscillator, a RF MEMS switch, or a MEMS sensor; infriction control; as a detector; and/or as a material for mechanicaluses in, such as, for example, milling, cutting, drilling, lacing, etc.

The following examples and comparative examples are for the purpose ofillustrating key elements but are not to be considered as limited by thefollowing examples.

Method of Characterization

Scanning Electron Microscopy (SEM) images were collected on a TescanVega scanning electron microscope equipped with an energy dispersiveanalysis X-Ray (EDAX) detector.

Raman spectra were collected by a Renishaw Raman Microscope (confocal).Laser Raman spectroscopy is widely used as a standard for thecharacterization of diamond, single crystal or polycrystalline. Itprovides readily distinguishable signatures of each of the differentforms (allotropes) of carbon (e.g., diamond, graphite, buckyballs,etc.). Combined with photoluminescence (PL) technology, it offers anon-destructive way to study various properties of diamond includingphase purity, crystal size and orientation, defect level and structure,impurity type and concentration, and stress and strain. In particular,the full-width-half-maximum, FWHM of the first order diamond Raman peakat 1332 cm⁻¹, as well as the Raman intensity ratio between diamond peakand graphitic peaks (D-band at 1350 cm⁻¹ and G-band at 1600 cm⁻¹), is adirect indicator of diamond quality. Furthermore, the stress and strainlevels in diamond grains and films can be estimated from diamond Ramanpeak shift. It has been reported that the diamond Raman peak shift rateunder hydrostatic stress is about 3.2 cm⁻¹/GPa, with the peak shiftingto lower wavenumber under tensile stress and higher wavenumber undercompressive stress. The Raman spectra presented herein were collectedusing a Renishaw inVia Raman spectroscope with 514 nm excitation laser.

Surface roughness (Ra) and Peak-to-Valley (PV) measurements of thesurface of a piece of polished diamond were obtained via a Zygo NewView600 Interferometer with a 20× Objective lens. The measured area was 200microns by 350 microns.

The scattering coefficient for 1.064 micron wavelength light wasmeasured and determined by the ratio of the scattered light intensitycollected at 34 mm distance from the sample (while the transmitted lightis blocked) to the total light intensity including the transmitted light(i.e., without blocking the transmitted light), using an Nd-YAG solidstate 1.064 micron wavelength laser.

Example 1: Producing Thin Diamond Substrate with One Surface Having anOptical Finish

A piece of single crystal silicon substrate 4 of 66 mm in diameter and11.5 mm in thickness was fabricated in a typical silicon fabricationprocess. Surface 12 of this silicon substrate 4 was diamond-turned to anoptical-finish surface with a Ra of 6 to 7 nm. At the same time, a logo“II-VI” was also machined recessively by diamond-turning onto surface 12of this silicon substrate 4. Then, this silicon substrate 4 was used asa substrate in microwave plasma CVD reactor 116 (FIG. 3), with thediamond-turned optically-finished surface (surface 12) facing in thedirection where plasma 120 will be formed.

A mixture of 1,850 mL/min hydrogen and 13.6 mL/min methane was flowedinto microwave plasma CVD reactor 116. After plasma ignition, themicrowave power and the reactor pressure were tuned so that the plasma120 size covered surface 12 of the silicon substrate 4. The diamondgrowth temperature at the center of surface 12 of silicon substrate 4was controlled, e.g., via optical pyrometer 126, to 800° C. After 168hours of diamond growth, the reaction was stopped. A polycrystallinediamond layer 6 of 285 microns cm thickness was conformingly depositedon surface 12 of the silicon layer 4. The silicon substrate 4 was thenstripped off with a KOH solution at elevated temperatures, followed byHF—HNO₃ stripping, resulting in a piece of free-standing diamond filmand with a diameter of 66 mm, a thickness of 285 microns, and an aspectratio of 231. The nucleation surface 10 of the diamond layer 6 wasmeasured to have a surface roughness (Ra) of 9.1 nm.

FIG. 4A is an image of this free-standing diamond layer 6 that isoptically clear with surface finishing on the growth side 14, and the“II-VI” logo that grew conformingly onto the diamond-turned trenches ofthe silicon surface 12, demonstrating that any non-planar surface of apiece of diamond can be attained by growing diamond conformingly intothe “Negative” surface of a sacrificial substrate (such as silicon layer4) that can be fabricated by a conventional fabrication process, suchas, for example, a diamond-turning process and an optics fabricationprocess that can involve grinding, lapping, ion etching and/or chemicalmechanical polishing. In an example, the non-planar, nucleation diamondsurface is of optical-finish. FIG. 4B is an enlarged view of thenucleation side 10 of the free-standing diamond layer 6 shown in FIG.4A. FIG. 4C is an enlarged view of the growth side 14 of thefree-standing diamond layer 6 shown in FIG. 4A.

A second piece of single crystal silicon substrate 4 of 66 mm indiameter and 11.5 mm in thickness was fabricated in a typical siliconfabrication process. Surface 12 of this silicon substrate 4 was alsodiamond-turned to an optical-finish surface with a Ra of 6 to 7 nm.Then, surface 12 of this silicon substrate 4 was ultrasonic-treated witha diamond-ethanol suspension slurry. This silicon substrate 4 was thenused as a substrate in microwave plasma CVD reactor 116 (FIG. 3), withthe diamond-turned optically-finished surface (surface 12) facing in thedirection where plasma 120 will be formed.

A mixture of 1,850 mL/min hydrogen and 13.6 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After plasma ignition, themicrowave power and the reactor pressure were tuned so that the plasma120 size covered the surface 12 of silicon substrate 4. The diamondgrowth temperature at the center of surface 12 of silicon substrate 4was controlled, e.g., via optical pyrometer 126, to 800° C. After 148hours of diamond growth the reaction was stopped. A polycrystallinediamond layer 6 of 233 microns in thickness was conformingly depositedon surface 12 of the silicon layer 4. The silicon substrate 4 was thenstripped off with a KOH solution at elevated temperatures, followed byHF—HNO₃ stripping, resulting in a piece of free-standing diamond film 6with a diameter of 66 mm, a thickness of 233 microns and, an aspectratio of 283. The nucleation surface 10 of the diamond film 6 wasmeasured to have a surface roughness (Ra) of 11.5 nm.

The quality of this latter diamond film 6 was studied via Ramanspectroscopy. As shown in FIG. 5, the diamond crystals on the growthside 14 are of excellent quality, as evidenced by a narrow FWHM of 2.8cm⁻¹ (as compared to the FWHM of 3.5 cm⁻¹ for a piece of single crystaldiamond), with the Raman shift centered between 1331.9 and 1332.1 cm⁻¹,suggesting no presence of stress in the diamond film on the growth side.The diamond crystals on the nucleation side are also very good, asevidenced by a Raman peak FWHM of 3.8 to 4.1 cm⁻¹, centered at 1331.6cm⁻¹ (indicating low stress in the diamond film on nucleation side).

Example 2: Producing Thin Diamond Substrate or Window with One Surfaceor Two Surfaces Having Optical Finishing

A piece of single crystal silicon substrate (or layer) 4 of 2 inches(50.8 mm) in diameter and 10 mm in thickness was fabricated in a typicalsilicon fabrication process. Both surfaces of this silicon substrate 4were optically finished via typical chemical-mechanical polishingprocess to an Ra of less than 1 nm. This silicon substrate 4 was thenultrasonically-treated with a diamond-ethanol suspension slurry andloaded in microwave plasma CVD reactor 116 (FIG. 3), with surface 12facing in the direction where plasma 120 will be formed.

A mixture of 1,850 mL/min hydrogen and 13.6 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After plasma 120 ignition,the microwave power and the reactor pressure were tuned so that theplasma 120 size covered surface 12 of silicon substrate.4 The diamondgrowth temperature at the center of surface 12 of silicon substrate 4was controlled, e.g., via optical pyrometer 126, to 800° C. After 140hours of diamond growth, the reaction was stopped resulting in apolycrystalline diamond film 6 of 200 to 220 microns in thicknessconformingly deposited on surface 12 of silicon substrate 4. The diamondgrowth surface 14, while the diamond layer 6 was still on the siliconlayer 4, was then lapped and polished to a surface roughness of Ra 5.0nm. After lapping and polishing, the thickness of diamond layer 6 onsilicon layer 4 was 125 microns. The silicon layer 4 was then strippedoff with a KOH solution at elevated temperatures, followed by HF—HNO₃stripping, resulting in a piece of free-standing diamond film 6 with adiameter of 2 inches (50.8 mm), a thickness of 125 microns, and anaspect ratio of 406. Both surfaces of this free-standing diamond film 6had an optically finish, applicable for uses as an optical window or asubstrate for other applications.

A second piece of single crystal silicon substrate (or layer) 4 of 2inches (50.8 mm) in diameter and 10 mm in thickness was fabricated in atypical silicon fabrication process. Both surfaces 12 and 16 of thissilicon substrate 4 were optically finished with a typicalchemical-mechanical polishing process to an Ra of less than 1 nm. Then,this silicon substrate 4 was ultrasonically-treated with adiamond-methanol suspension slurry. This silicon substrate 4 was thenloaded into microwave plasma CVD reactor 116, with surface 12 facing inthe direction where plasma 120 will be formed.

A mixture of 2,700 mL/min hydrogen and 16.2 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After plasma 120 initiation,the microwave power and the reactor pressure were tuned so that theplasma 120 size covered surface 12 of silicon substrate 4. The diamondgrowth temperature at the center of surface 12 was controlled, e.g., viaoptical pyrometer 126, to between 832° C. and 866° C. After 72 hours ofdiamond growth, the reaction was stopped resulting in a polycrystallinediamond film 6 of 110 to 130 microns in thickness conformingly depositedon surface 12 of silicon substrate 4. The diamond growth surface 14,while diamond film 6 was still on the silicon substrate 4, was thenlapped and polished to a surface roughness (Ra) of 5.8 nm. After lappingand polishing, the thickness of diamond film 6 on silicon substrate 4was 60 to 70 microns. The silicon substrate 4 was then stripped off witha KOH solution at elevated temperatures, followed by HF—HNO₃ stripping,resulting in a piece of free-standing diamond film 6 with a diameter of2 inches, a thickness of 60-70 microns, and an aspect ratio of 781. Bothsurfaces of this free-standing diamond 6 had an optically finish,applicable for uses as an optical window or a substrate otherapplications.

A third piece of single crystal silicon substrate (or layer) 4 of 2inches (50.8 mm) in diameter and 10 mm in thickness was fabricated in atypical silicon fabrication process. Both surfaces 12 and 16 of thissilicon substrate 4 were optically finished with a typicalchemical-mechanical polishing process to an Ra of less than 1 nm. Then,silicon substrate 4 was rubbed with a 0.25 micron diamond slurry to anoptical finish, followed by a typical cleaning process. This siliconsubstrate 4 was then loaded into microwave plasma CVD reactor 116, withthe diamond-slurry-rubbed optically-finished surface 12 facing in thedirection where plasma 120 will be formed.

A mixture of 2,700 mL/min hydrogen and 16.2 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After plasma 120 ignition,the microwave power and the reactor pressure were tuned so that theplasma 120 size covered the surface 12 of silicon substrate 4. Thediamond growth temperature at the center of surface 12 was controlled,e.g., via optical pyrometer 120, to between 794 and 835° C. After 95hours of diamond growth, the reaction was stopped resulting in apolycrystalline diamond film 6 of 156 microns in thickness conforminglydeposited on surface 12 of the silicon substrate 4. The diamond film 6harvested (by removing silicon layer 4) had an aspect ratio of 326. Thesurface roughness of the nucleation side 10 was 7.7 nm and the surfaceof the growth side 14 of the diamond film 6 was rough, as is typical foran as-grown diamond surface.

Example 3: Diamond Growth on a Piece of 166 mm×10 mm Single Crystal SiSubstrate with Both Surfaces Chemically-Etched, Producing Thin DiamondSubstrate(s) with One Surface Having Optical Finishing

A piece of single crystal silicon substrate 4 of 166 mm in diameter and10 mm in thickness was fabricated in a typical silicon fabricationprocess. Both surfaces 12 and 16 of this silicon substrate 4 werefinished with a typical chemical-etching process. Then, this siliconsubstrate 4 was rubbed with diamond powders and loaded into microwaveplasma CVD reactor 116.

A mixture of 2,800 mL/min hydrogen and 84 mL/min methane was flowed intothe microwave plasma CVD reactor 116. After plasma 120 ignition, themicrowave power and the reactor pressure were tuned so that the plasmasize covered surface 12 of the silicon substrate 4. The diamond growthtemperature at the center of surface 12 was controlled, e.g., viaoptical pyrometer 126, to 1120° C. After 44 hours of diamond growth, thereaction was stopped resulting in a composite substrate 2 comprisingpolycrystalline diamond film 6 of 350 microns in thickness conforminglydeposited on the surface 12 of silicon substrate 4. The diamond growthsurface 14, while the diamond was still on the silicon substrate 4, wasthen lapped flat. After lapping, the diamond film 6 thickness was 300microns. Then, the exposed silicon surface 16 of this piece ofdiamond-on-silicon substrate 2 was lapped until the total thickness ofthis composite substrate 2 was 1.7-1.8 mm.

The lapped diamond surface 14 was then further polished to an opticalfinish, producing a thin diamond film (i e, thinner than 300 micron inthickness) on a silicon substrate 4 of 166 mm in diameter. Thiscomposite substrate 2 can be used as an optical mirror, or as asubstrate for electronics, photonics, or optoelectronics. The siliconsubstrate 4 was then stripped off, producing a free standing thindiamond film 6 with a growth surface optically finished, having anaspect ratio of ≥533.

Prior to removal of silicon substrate 4, a piece 50 mm in diameter andseveral pieces 1 inch in diameter were laser-cut from thediamond-on-silicon composite substrate 2 for polishing the diamondgrowth surface 14. The growth surface 14 roughness of the 50 mm piecereached 1 nm; and the finished diamond layer 6 thickness was estimatedto be 170-180 microns. The silicon substrate 4 was then be stripped offwith a KOH solution at elevated temperatures, followed by HF—HNO₃stripping, resulting in a free-standing diamond layer 6 with a diameterof 50 mm in diameter, a thickness of 175 microns, and an aspect ratio of285. Its diamond growth surface 14 was optically-finished, applicablefor uses as an optical mirror or a substrate for other applications suchas thermal management. Alternatively, to removing the silicon substrate4, the silicon substrate 4 can be thinned and surface 16 polished sothat a piece of diamond-silicon composite substrate 2 with at least thediamond surface 14 that is finished optically is obtained.

Example 4: Diamond Growth on a Piece of 166 mm×10 mm Polycrystalline SiSubstrate with Both Surfaces Chemically-Etched, Producing Thin DiamondSubstrate(s) with One Surface Having Optical Finishing

A piece of polycrystalline silicon substrate 4 of 166 mm in diameter and10 mm in thickness was fabricated via a typical silicon fabricationprocess. Both surfaces 12 and 16 of this piece of silicon substrate 4were finished with a typical chemical-etching process. Then, this pieceof silicon substrate 4 was rubbed with diamond powders and then loadedinto microwave plasma CVD reactor 116.

A mixture of 2,800 mL/min hydrogen and 84 mL/min methane was flowed intothe microwave plasma CVD reactor 116. After plasma 120 ignition, themicrowave power and the reactor pressure were tuned so that the plasmasize covered surface 12 of the silicon substrate 4. The diamond growthtemperature at the center of surface 12 was controlled, e.g., viaoptical pyrometer 126, to 1120° C. After 24 hours of diamond growth, thereaction was stopped resulting in a polycrystalline diamond film 6 of175 microns in thickness conformingly deposited on the surface 12 of thesilicon substrate 4. The diamond growth surface 14, while the diamondwas still on the silicon substrate 4, was then lapped flat. The lappeddiamond growth surface 14 was further polished to an optical finish. Thesilicon substrate 4 was then stripped off with a KOH solution atelevated temperatures, followed by HF—HNO₃ stripping, resulting in afree-standing diamond film 6 with a diameter of 166 mm, a thickness ofless than 175 microns, and an aspect ratio of 948. The diamond growthsurface 14 that was optically-finished, can be used as an optical mirroror can be used as a substrate for other applications such as thermalmanagement. Alternatively to removing silicon substrate 4, the siliconsubstrate 4 (while the diamond layer 6 is still attached) can optionallybe thinned and surface 16 polished so that a piece of diamond-siliconcomposite substrate 2 with at least the diamond surface finishedoptically is obtained, which can be useful as an optical mirror, or asubstrate for electronics, photonics, or optoelectronics.

Example 5: Diamond Growth on a Piece of 166 mm×10 mm Polycrystalline SiSubstrate with One Surface Optically-Finished (Ra 1.3 nm) by a ChemicalMechanical Polishing Process, Producing a Thin Diamond Window orSubstrate with One Surface or Both Surfaces Optically Finished

A piece of polycrystalline silicon substrate 4 of 166 mm in diameter and10 mm in thickness was fabricated in a typical silicon fabricationprocess. Both surfaces 12 and 16 of this piece of silicon substrate 4were finished via a typical chemical-etching process. Then, surface 12was polished to an optical finish (Ra 1.3 nm) with a chemical mechanicalpolishing process. This piece of silicon substrate 4 was then treatedultrasonically with an aqueous diamond slurry and then loaded intomicrowave plasma CVD reactor 16 with surface 12 facing in the directionwhere plasma 120 will be formed.

A mixture of 2,800 mL/min hydrogen and 16.8 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After plasma 120 ignition,the microwave power and the reactor pressure were tuned so that theplasma size covered surface 12 of silicon substrate 4. The diamondgrowth temperature at the center of surface 12 was controlled, e.g., viaoptical pyrometer 126, to between 846 and 868° C. After 164 hours ofdiamond growth, the reaction was stopped resulting in a polycrystallinediamond film 6 of 295 microns in thickness conformingly deposited on thesurface 12 of the silicon substrate 4. The diamond growth surface 14,while the diamond layer 6 was still on the silicon layer 4, was thenlapped flat. The lapped diamond surface 14 was further polished to anoptical finish, which resulted in diamond layer 6 having an aspect ratioof 563.

The silicon layer 4 side of this diamond-silicon composite substrate 2was ground to a total thickness of 1.7-2.0 mm. The diamond-siliconcomposite substrate 2 was then laser-cut to pieces of 1″ (25.4 mm) indiameter and 85 mm in diameter, followed by optical polishing of thegrowth surface 14 of diamond layer 6 to attain an optically-finishedsurface with a diamond layer 6 thickness ≤150 microns.

In an example, the silicon layer 4 can then be stripped off with a KOHsolution at elevated temperatures, followed by HF—HNO₃ stripping,resulting in free-standing diamond layers 6 of 1″ (25.4 mm) and 85 mm indiameter, a thickness ≤150 microns, and an aspect ratio of 566. Thediamond growth surface 14 that is optically-finished can be used as anoptical mirror or can be used as a substrate for other applications suchas thermal management. Alternatively to removing silicon substrate 4,the silicon substrate 4 (while the diamond layer 6 is still attached)can be thinned and surface 16 polished so that a piece ofdiamond-silicon composite substrate 2 with at least an opticallyfinished diamond surface is obtained, which can be useful as an opticalmirror, or as a substrate for electronics, photonics, optoelectronics,etc.

Comparative Example 1: Failures in Diamond Growth on Silicon Wafers (6″in Diameter and 625 Microns in Thickness)

Three silicon substrates or wafers 4 (n-type, 6 inches (152.4 mm) indiameter and 625 microns in thickness) were provided. Surface 12 of eachsilicon wafer 4 was chemically-mechanically polished to an opticalsurface finish and the other surface 16 of each silicon wafer 4 wasfinished with chemical etching. Surface 12 (optically finished) of eachsilicon wafer 4 was then rubbed with diamond powders and the siliconwafer 4 was placed inside microwave plasma CVD reactor 116 for diamondgrowth, with the optically-finished surface 12 facing in the directionwhere plasma 120 will be formed.

A mixture of 2,500 mL/min hydrogen and 75 mL/min methane was flowed intothe microwave plasma CVD reactor 116. After plasma 120 ignition, themicrowave power and the reactor pressure were tuned so that the plasmasize covered surface 12 of the silicon wafer 4. During this plasmatuning process, the silicon wafer 4 shattered into a number of smallerpieces. This experiment was repeated with the other two silicon wafers4, varying the rate of the power and pressure increases, with the sameresult, namely, the other two silicon wafers also shattered into anumber of smaller pieces.

Comparative Example 2: Failures in Attaining Optically-Finished Diamondthat is Thinner than 400 Microns in Thickness

Pieces of diamond with diameters of 50 mm, 75 mm, 85 mm, and 100 mm werelaser-cut from an as-grown diamond wafer with a thickness greater than550 microns. In an example, this as-grown diamond wafer was growndirectly on substrate holder 122 of microwave plasma CVD reactor 116without having a silicon layer 4 between the as-grown diamond wafer andsubstrate holder 122. Then, the growth surface of each diamond piece waslapped flat. Then, one side of each surface, either the growth surfaceor the nucleation surface, of these diamond pieces were polished to anoptical-finish by a conventional polishing technique. When flipping overand trying to thin and polish the other surface, these diamond piecesshattered before approaching 400 microns in thickness, suggesting thatan aspect of ratio of 125 is difficult to achieve with conventionaldiamond polishing processes.

Example 6: Producing Thin Diamond Coating on Silicon Substrate with BothSurfaces Having Optical Finishing

A polycrystalline silicon substrate 4 of 75 mm in diameter and 10 mm inthickness was used as substrate for diamond CVD growth of a diamondlayer 6. Surface 12 of this silicon substrate 4 was polished to a mirrorfinish with roughness Ra<1 nm. In order to achieve high diamondnucleation density on the mirror finished silicon surface 12, while alsoobtaining good diamond-to-silicon adhesion to prevent the diamond layerfrom delaminating from this silicon substrate 4, a two-step seedingprocess was employed. Firstly, this silicon substrate 4 was treated with0.25 μm average size diamond powder/methanol suspension solution in anultrasonic bath. Then, a first step diamond nucleation process onsurface 12 was carried out on silicon substrate 4 for one hour inmicrowave plasma CVD reactor 116 whereupon silicon substrate 4 wasremoved from microwave plasma CVD reactor 116. This nucleation stepresulted in relatively low density diamond nucleation at <10⁵/cm².

FIG. 6 is an SEM photo of the silicon surface after the first nucleationstep.

Next, this silicon substrate 4 (after the first nucleation step) wastreated with nano-crystal diamond powder/methanol suspension solution inultrasonic bath. Then, this silicon substrate 4 was re-loaded into themicrowave plasma CVD reactor 116 for a second step nucleation andconsecutive diamond growth on silicon surface 12.

To this end, a mixture of 2,400 mL/min hydrogen and 16.8 mL/min methanewas flowed into the microwave plasma CVD reactor 116 in the first stepand second step diamond nucleation process. After initiating the plasma130, the microwave power and the reactor pressure were tuned so that theplasma size covered the surface 12 of the silicon substrate 4. Thediamond growth temperature at the center of surface 12 was controlled,via control pyrometer 126, at 800° C. by cooling substrate holder 4.After 14 hours of diamond growth in the second step nucleation, thereaction was stopped, resulting in a polycrystalline diamond film 6 of20 μm thickness conformingly deposited on surface 12 of the siliconsubstrate 4. The diamond growth surface 14, while still on the siliconsubstrate 4, was then polished until diamond film 6 had a thickness of10 μm and a surface 14 roughness of avg. Ra=2.5 nm (as shown in FIG. 7).

The surface roughness of the diamond nucleation side 10 at thesilicon-diamond interface 8 was measured on another diamond sampleformed using the same 2-step seeding process and after siliconstripping. The average surface roughness was measured to be avg. Ra=1.7nm (as shown in FIG. 8). The diamond nucleation density was estimatedunder SEM to be >10⁹/cm⁻¹ after the second nucleation step (as seen inFIG. 9). The flatness of the polished diamond growth surface 14 wasstudied under optical interferometer. The diamond growth surface 14 wasdetermined to be slightly convex, with a peak-to-valley heightdifference across entire 75 mm diameter surface of 1.53 μm, and anirregularity of 0.53 μm (as shown in FIG. 10).

Diamond quality and phase purity were evaluated using a Ramanspectroscope. FIG. 11 shows Raman spectra collected from (a) theas-grown diamond growth surface 14 and (b) polished diamond growthsurface 14 with 514 nm excitation laser. Sharp diamond peaks at 1332cm⁻¹ with peak width (FWHM) of 4.3-6.2 cm⁻¹ were observed with nodetectable sp² carbon signals, meaning a high quality diamond layer 6was deposited on silicon layer 4. A strong peak also presents at 520cm⁻¹, which is a silicon Raman peak. This silicon peak comes from thesilicon substrate or layer 4 underneath diamond layer 6.

Example 7: Producing Thin Diamond Coating on Silicon Substrate with OneSurface Having Optical Finishing

A polycrystalline silicon substrate 4 of 75 mm in diameter and 10 mm inthickness was used as substrate for diamond CVD growth of a diamondlayer 6. Both surfaces 12 and 16 of this silicon substrate 4 werechemically etched. This silicon substrate 4 was then treated withnano-crystalline diamond powder/methanol suspension solution in anultrasonic bath to promote diamond nucleation. The silicon substrate wasthen loaded into microwave plasma CVD reactor 116.

A mixture of 2,400 mL/min hydrogen and 16.8 mL/min methane was flowedinto the microwave plasma CVD reactor 116. After initiating the plasma120, the microwave power and the reactor pressure were tuned so that theplasma 120 size covered the surface 12 of the silicon substrate 4 facingthe plasma 120. The diamond growth temperature at the center of thesurface 12 of the silicon substrate 4 facing the plasma 120 wascontrolled, via optical pyrometer 126, at 800° C. by cooling substrateholder 122. After 10 hours of diamond growth, the reaction was stoppedresulting in a polycrystalline diamond film 6 of 14 microns thicknessconformingly deposited on the surface 12 of the silicon substrate 4. Thediamond growth surface 14, while still on the silicon substrate 4, waspolished until diamond layer 6 had a thickness of 8 microns and asurface roughness of avg. Ra=3.0 nm.

Example 8. Diamond Growth on an Optically Finished PolycrystallineSilicon Substrate (166 mm Diameter×10 mm Thickness) by 0.25 mm DiamondParticle Seeding

In another example, a polycrystalline silicon substrate 4 of 166 mm indiameter and 10 mm in thickness was used as a substrate for CVD growthof diamond layer 6. Both surfaces 12 and 16 of this silicon substrate 4were finished via a typical chemical-etching process. Surface 12 of thissilicon substrate 4 was then chemically-mechanically polished to amirror finish with roughness Ra<1.5 nm. The entire silicon substrate 4was then treated (seeded) ultrasonically in a ultrasonic bath in a 0.25μm average size diamond powder/methanol suspension solution and thenplaced in a CVD reactor 116 (FIG. 2) with surface 12 facing quartzwindow 118 (FIG. 3).

A mixture of 2,800 mL/min hydrogen and 16.8 mL/min methane was thenflowed into CVD reactor 116 under the control of mass flow controller108. After plasma 120 ignition, the microwave 114 power and the reactor116 pressure were tuned so that the plasma 120 size covered surface 12of silicon substrate 4. The diamond growth temperature at the center ofsubstrate was controlled, e.g., via optical pyrometer 126, to between845 and 868° C. After 163 hours of diamond growth, the diamond growthreaction was stopped resulting in a silicon-diamond composite with apolycrystalline diamond film 6 thickness of 295 μm conforminglydeposited on surface 12 of silicon substrate 4—an aspect ratio of 563.

While diamond film 6 was adhered to silicon substrate 4, the growthsurface of diamond film 6 was polished to an optically finish (Rabetween 3-5 nm) and a thickness (of diamond film 6) of 99 microns—anaspect ratio of 168. While diamond film 6 was still adhered to siliconsubstrate 4, the silicon-diamond composite was laser-cut to a number ofdifferent pieces having different diameters, followed by removal of thesilicon substrate 4 (dissolved by KOH solution) of each piece therebyforming pieces of free-standing diamond film 6. The nucleation surfacesof these pieces of free-standing diamond film 6 had an average surfaceroughness (Ra) between 5 and 9 nm, and a nucleation density of ≥10⁵/cm².One of these pieces of free-standing diamond film 4 had a thickness of99 microns and was characterized for 1.06 μm wavelength light scatteringwith a light scattering coefficient of 8.22/cm at a distance of 34.0 mmfrom a scattering light collection lens.

Example 9. Growth Diamond on Optically Finished Silicon Substrate (2Inch (50.8 mm) Diameter×10 mm in Thickness) by 1^(st) Step Seeding (0.25mm Diamond Particle Seeding) and Followed by 2^(nd) Step Seeding(Nano-Diamond Particle Seeding)

In another example, a polycrystalline silicon substrate 4 of 50.8 mm indiameter and 10 mm in thickness was used as a substrate for CVD growthof diamond layer 6. Both surfaces 12 and 16 of this silicon substrate 4where were chemically-mechanically polished to a mirror finish withroughness Ra<1.5 nm. The entire silicon substrate 4 was thenultrasonically treated (seeded) in a 0.25 μm average size diamondpowder/methanol suspension solution and then placed in a CVD reactor 116(FIG. 3). This silicon substrate 4 was then loaded into microwave plasmaCVD reactor 116.

A mixture of 2,800 mL/min hydrogen and 16.8 mL/min methane was thenflowed into CVD reactor 116 under the control of mass flow controller 8.After plasma 120 ignition, the microwave 114 power and the reactor 116pressure were tuned so that the plasma 120 size covered surface 12 ofsilicon substrate 4 that faced quartz window 118. The diamond growthtemperature at the center of silicon substrate 4 was controlled, e.g.,via optical pyrometer 126, to 780° C. After one hour of diamond growth,the diamond growth reaction was stopped, and the diamond-seeded siliconsubstrate 4 was observed to have diamond particles deposited aboutsurface 12 of silicon substrate 4.

The diamond-seeded silicon substrate 4 was then removed from CVD reactor116 and ultrasonically treated in a nano-crystal diamond powder (typicalparticle size <20 nm)/methanol suspension solution in a ultrasonic bath.The nano-diamond-treated diamond-seeded Si substrate was then reloadedinto CVD reactor 116 with surface 12 again facing plasma 120. A mixtureof 2,800 mL/min hydrogen and 16.8 mL/min methane was then flowed intoCVD reactor 116 under the control of mass flow controller 8. Afterplasma 120 ignition, the microwave 114 power and the reactor 116pressure were tuned so that the plasma 120 size covered surface 12 ofsilicon substrate 4. The diamond growth temperature at the center of thesilicon substrate 4 was controlled, e.g., via optical pyrometer 126, tobetween 790 and 821° C.

After an additional 143 hours of diamond growth, the diamond growthreaction was stopped resulting in a silicon-diamond composite with adiamond film 6 thickness of 245 microns—an aspect ratio of 207. Thediamond growth surface, while diamond film 6 was still on siliconsubstrate 4, was polished to an optical finish (Ra between 3-5 nm) and athickness (of diamond film 6) of 197 microns—an aspect ratio of 258.

Silicon substrate 4 was then removed (dissolved with a KOH solution)from this silicon-diamond composite leaving a free-standing diamond film6. The nucleation surface of this free-standing diamond film 6 had anaverage surface roughness (Ra)=2.73 nm, a nucleation density of≥10⁹/cm², and a smooth surface finish, the latter two of which arehighly desirable for applications such as, for example, thermalmanagement, optical management, semiconductor device, friction control,acoustic wave management, etc. This free-standing diamond film 6 wasalso characterized for 1.06 μm wavelength light scattering with a lightscattering coefficient of 2.69/cm at a distance of 34 mm from thescattering light collection lens. 1.06 μm wavelength light scattering isconsidered in the art as low micron light scattering and is highlydesirable for optical, thermal, acoustic applications, etc.

Example 10. Growth Diamond on Optically Finished Silicon Substrate (2Inch (50.8 mm) Diameter×10 mm in Thickness) by One Step Seeding(Nano-Diamond Particle Seeding)

In another example, a polycrystalline silicon substrate 4 of 50.8 mm indiameter and 10 mm in thickness was used as a substrate for CVD growthof diamond layer 6. Surface 12 of this silicon substrate 4 waschemically-mechanically polished to a mirror finish with roughnessRa<1.5 nm, while the other surface 16 was etched via a typicalchemical-etching process. Next, this silicon substrate 4 wasultrasonically treated in a nano-crystal diamond powder (typicalparticle size <20 nm)/methanol suspension solution in a ultrasonic bath.This silicon substrate was then loaded into CVD reactor 116 with Surface12 facing quartz window 118.

A mixture of 2,800 mL/min hydrogen and 16.8 mL/min methane was thenflowed into CVD reactor 116 under the control of mass flow controller108. After plasma 120 ignition, the microwave 114 power and the reactor116 pressure were tuned so that the plasma 120 size covered surface 12of silicon substrate 4. The diamond growth temperature at the center ofsilicon substrate 4 was controlled, e.g., via optical pyrometer 126, to800° C.

After 118 hours of diamond growth, the diamond growth reaction wasstopped resulting in a silicon-diamond composite with a diamond film 6thickness of 190 μm conformingly deposited on surface 12 of siliconsubstrate 4. The diamond growth surface, while diamond film 6 was stillon silicon substrate 4, was polished to an optical finish and athickness (of diamond film 6) of 140 microns.

Silicon substrate 4 was then removed (dissolved with a KOH solution)from this silicon-diamond composite leaving a free-standing diamond film6. The nucleation surface of this free-standing diamond film 6 had anaverage surface roughness (Ra) between 2 and 3 nm, a nucleation densityof ≥10⁹/cm², and a smooth surface finish, the latter two of which arehighly desirable for applications such as, for example, thermalmanagement, optical management, semiconductor device, friction control,etc.

This free-standing diamond film 6 was also characterized for 1.06 μmwavelength light scattering with a light scattering coefficient at2.09/cm at a distance 34 mm from the scattering light collection lens.

Several additional runs in accordance with the principles of thisExample 10 on different silicon substrates 4 under the same diamondgrowth conditions experienced problems. In one example, diamond film 6delaminated from silicon substrate 4, which did not allow for furtherpolishing to be performed on the growth surface of the delaminateddiamond film. These examples suggest that nano-diamond seeding alone maynot be a reliable process for producing a thin diamond substrate with atleast one optically finished surface. This suggests that the diamondfilm 6 is not strongly adhered to silicon substrate 4 with nano-diamondseeding process.

In an example, a dielectric film 13 can be included on the surface 12 ofany one or more of the substrates described herein as a possible aid toadhesion of diamond film 6 on the substrate via the dielectric film 13.However, this is not to be construed in a limiting sense.

As can be seen, disclosed herein is a multilayer composite substratecomprising a silicon layer having an optically finished surface and achemical vapor deposition (CVD) grown diamond layer on the opticallyfinished surface of the silicon layer, wherein at an interface of thesilicon layer and the diamond layer, the optically finished surface ofthe silicon layer has a surface roughness (Ra)≤100 nm.

The multilayer composite substrate can optionally include a dielectriclayer or film between the silicon layer and the diamond layer. Thedielectric film can be SiO₂ or SiN.

At the interface of the silicon layer and the diamond layer, theoptically finished surface of the silicon layer can have an Ra≤1 nm.

At the interface of the silicon layer and the diamond layer, a surfaceof the diamond layer can have an Ra no less than the Ra of the opticallyfinished surface of the silicon layer.

At the interface of the silicon layer and the diamond layer, a graindensity of the diamond can be ≥10⁹ diamond grains/cm² or ≥10⁴ diamondgrains/cm².

The silicon layer can have a thickness ≥5000 μm or a thickness ≥10 μm.

The diamond layer can have a thickness ≤2000 μm or a thickness ≤5 μm.

The multilayer substrate can have a largest dimension ≥25.4 mm or alargest dimension ≥152.4 mm.

The multilayer substrate can have a total thickness of the silicon layerand the diamond layer ≥50 μm or a total thickness of the silicon layerand the diamond layer ≥5 mm.

The surface of the diamond layer opposite the silicon layer can have anRa≤100 nm or an Ra≤5 nm.

A light management coating can be included on a surface of the diamondlayer opposite the silicon layer.

Diamond crystals on a surface of the diamond layer opposite the siliconlayer can have a Raman peak between 1331.9 and 1332.1 cm-1, a full widthat half maximum (FWMH) of the Raman spectrum between 2.8 and 6.2 cm-1,or both.

Also disclosed is a method of forming the multilayer substrate of claim1, comprising: (a) providing a silicon layer having the opticallyfinished surface; (b) following step (a), seeding the optically finishedsurface of the silicon layer with diamond particles; (c) following step(b), CVD depositing a diamond layer on the diamond seeded opticallyfinished surface of the silicon layer; and (d) continue CVD depositingthe diamond layer on the diamond seeded optically finished surface ofthe silicon layer until a predetermined thickness of the diamond layeris deposited on the diamond seeded optically finished surface of thesilicon layer.

Between steps (c) and (d), the method can include the steps of: (c1)stopping CVD depositing the diamond layer on the diamond seededoptically finished surface of the silicon layer; and (c2) seeding theoptically finished surface of the silicon layer including the CVDdeposited diamond layer of step (c) with diamond particles.

The silicon layer of step (a) can include a substrate bonded to a sideof the silicon layer opposite the optically finished surface. Thesubstrate can be thicker than the silicon layer.

The method can further include, (e) polishing a surface of the diamondlayer opposite the silicon layer to an optical finish.

The method can further include applying a light management coating onthe polished surface of the diamond layer opposite the silicon layer.

While the foregoing examples have been described with reference to amultilayer composite substrate comprising a silicon layer having achemical vapor deposition (CVD) grown diamond layer deposited on asurface of the silicon layer, it is also envisioned that a multilayercomposite substrate in accordance with the principals described hereincan include a diamond layer CVD grown on a surface of any suitableand/or desirable lattice matched substrate.

In an example, at 300° K diamond has a lattice constant of 3.57angstrom, and silicon has a lattice constant 5.43 angstrom. Latticeconstants of other elements at 300° K are known in the art and will notbe described herein for brevity.

In an example, diamond layer 6 can be grown on a substrate made of anysuitable and/or desirable material or combination of materials. Saidsubstrate can be crystalline or amorphous (non-crystallized)material(s). In an example, when said substrate is made from crystallinematerial(s), the absolute lattice mismatch between diamond layer 6 andsaid substrate, i.e., the difference between the lattice constant of thediamond layer and the lattice constant of the substrate, can be from 0to 4.5 angstrom. Herein, “lattice matched substrate” means that thatabsolute lattice mismatch between the substrate (of any material(s)) anddiamond layer 6, i.e., the difference between the lattice constant ofthe diamond layer and the lattice constant of the substrate, is between0 and 4.5 angstrom, e.g., ≤4.5 angstrom, ≤3.5 angstrom, ≤2.5 angstrom,or ≤2.0 angstrom. It is believed that a diamond layer 6 can besuccessfully CVD grown in accordance with the principals describedherein on a surface of one or more lattice matched substrates known inthe art.

The foregoing examples have been described with reference to theaccompanying figures. Modifications and alterations will occur to othersupon reading and understanding the foregoing examples which are providedfor the purpose of illustration and are not to be construed in alimiting sense. Accordingly, the foregoing examples are not to beconstrued as limiting the disclosure.

The invention claimed is:
 1. A method of forming a multilayer substrate,the multilayer substrate comprising a silicon layer having an opticallyfinished surface and a multilayer chemical vapor deposition (CVD) growndiamond film on the optically finished surface of the silicon layerincluding nanodiamond particles between at least one pair of CVD layers,wherein at an interface of the silicon layer and the diamond film, theoptically finished surface of the silicon layer has a surface roughness(Ra)≤100 nm, the method comprising: (a) providing a silicon layer havingthe optically finished surface; (b) following step (a), seeding theoptically finished surface of the silicon layer with diamond particles;(c) following step (b), CVD depositing a diamond layer on a diamondseeded optically finished surface of the silicon layer; and (d) continueCVD depositing the diamond layer on the diamond seeded opticallyfinished surface of the silicon layer until a minimum thickness of thediamond layer is deposited on the diamond seeded optically finishedsurface of the silicon layer.
 2. The method of claim 1, furtherincluding, between steps (c) and (d), the steps of: (c1) stopping CVDdepositing the diamond layer on the diamond seeded optically finishedsurface of the silicon layer; and (c2) seeding the optically finishedsurface of the silicon layer including the CVD deposited diamond layerof step (c) with diamond particles.
 3. The method of claim 1, wherein:the silicon layer of step (a) includes a substrate bonded to a side ofthe silicon layer opposite the optically finished surface; and thesubstrate is thicker than the silicon layer
 4. The method of claim 1,further including: (e) polishing a surface of the diamond layer oppositethe silicon layer to an optical finish.
 5. The method of claim 4,further including applying a light management coating on the polishedsurface of the diamond layer opposite the silicon layer.
 6. The methodof claim 1, wherein the minimum thickness of step (d) is 10 microns.